U.S. patent application number 13/968129 was filed with the patent office on 2015-02-19 for systems and methods for photoacoustic spectroscopy.
This patent application is currently assigned to Covidien LP. The applicant listed for this patent is Covidien LP. Invention is credited to Charles Haisley, Sarah Hayman, Qiaojian Huang, Youzhi Li, Friso Schlottau.
Application Number | 20150051473 13/968129 |
Document ID | / |
Family ID | 52467294 |
Filed Date | 2015-02-19 |
United States Patent
Application |
20150051473 |
Kind Code |
A1 |
Huang; Qiaojian ; et
al. |
February 19, 2015 |
SYSTEMS AND METHODS FOR PHOTOACOUSTIC SPECTROSCOPY
Abstract
A photoacoustic sensor system includes a photoacoustic sensor
assembly having a light emitting component configured to emit one
or more wavelengths of light into a region of a patient's tissue
and an acoustic detector configured to detect acoustic energy
generated within the region of the patient's tissue in response to
the emitted light. The photoacoustic sensor assembly is configured
to generate a signal that enables a monitor to determine a potion
of the sensor assembly relative to the patient's tissue.
Inventors: |
Huang; Qiaojian;
(Broomfield, CO) ; Haisley; Charles; (Boulder,
CO) ; Hayman; Sarah; (Boulder, CO) ; Li;
Youzhi; (Longmont, CO) ; Schlottau; Friso;
(Lyons, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Covidien LP |
Mansfield |
MA |
US |
|
|
Assignee: |
Covidien LP
Mansfield
MA
|
Family ID: |
52467294 |
Appl. No.: |
13/968129 |
Filed: |
August 15, 2013 |
Current U.S.
Class: |
600/407 |
Current CPC
Class: |
A61B 5/0095 20130101;
A61B 5/6815 20130101 |
Class at
Publication: |
600/407 |
International
Class: |
A61B 5/00 20060101
A61B005/00; A61B 5/1455 20060101 A61B005/1455; A61B 5/024 20060101
A61B005/024 |
Claims
1. A photoacoustic system, comprising: a photoacoustic sensor
assembly, comprising: a light emitting component configured to emit
one or more wavelengths of light into a region of a patient's
tissue; and an acoustic detector configured to detect acoustic
energy generated within the region of the patient's tissue in
response to the emitted light, wherein the sensor assembly is
configured to generate a signal that enables a monitor to determine
a position of the sensor assembly relative to the patient's
tissue.
2. The photoacoustic system of claim 1, comprising an electrical
contact sensor configured to generate the signal.
3. The photoacoustic system of claim 2, wherein the electrical
contact sensor is positioned outside of a pathway through which the
acoustic energy is transmitted from the patient's tissue to the
acoustic detector.
4. The photoacoustic system of claim 1, comprising an optical
detector configured to receive light emitted by the light emitting
element and to generate the signal based on the received light.
5. The photoacoustic system of claim 1, wherein the light emitting
component is oriented such that emitted light contacts the
patient's tissue at a non-orthogonal angle with respect to the
patient's tissue when the sensor is applied to the patient's
tissue.
6. The photoacoustic system of claim 1, comprising the monitor
coupled to the photoacoustic sensor assembly, wherein the monitor
comprises a memory storing instructions for: receiving the signal
from the photoacoustic sensor assembly; determining the position of
the photoacoustic sensor assembly based at least in part on the
signal received from the photoacoustic sensor assembly; and
adjusting a light drive signal to the light emitting component
based at least in part on the position.
7. The photoacoustic system of claim 6, wherein the adjusting
comprises reducing an intensity of the light emitted by the light
emitting component.
8. The photoacoustic system of claim 6, further comprising
providing an indication of the position.
9. The photoacoustic system of claim 1, wherein the signal
comprises the detected acoustic energy.
10. A photoacoustic monitoring system, comprising: a photoacoustic
sensor assembly configured to emit light; a monitor comprising: a
memory storing instructions for: receiving a signal from the sensor
assembly indicative of a proximity of the sensor assembly to a
tissue of a patient; determining whether the sensor assembly is
positioned on the tissue of the patient based on the signal; and
adjusting the emitted light based at least in part on the
determination; and a processor configured to execute the
instructions.
11. The photoacoustic monitoring system of claim 10, wherein the
sensor assembly comprises an optical detector configured to receive
the emitted light and to generate the signal indicative of the
proximity of the sensor assembly to the tissue of the patient,
wherein the memory stores instructions for qualifying the signal to
determine whether the sensor assembly is applied to the tissue of
the patient.
12. The photoacoustic monitoring system of claim 10, wherein the
photoacoustic sensor assembly comprises an acoustic detector
configured to detect the emitted light after the emitted light
passes through the patient's tissue and to generate an acoustic
signal in response to the detected light, wherein the signal
indicative of the proximity of the sensor assembly to the tissue of
the patient comprises the acoustic signal.
13. The photoacoustic monitoring system of claim 10, wherein the
memory stores instructions for increasing the power if it is
determined that the sensor assembly is applied to the tissue of the
patient.
14. The photoacoustic monitoring system of claim 10, wherein the
memory stores instructions for increasing the pulse width of the
emitted light if it is determined that the sensor assembly is
applied to the tissue of the patient.
15. The photoacoustic monitoring system of claim 10, wherein the
memory stores instructions for disabling the light emitting
component if it is determined that the sensor assembly is not
applied to the tissue of the patient.
16. The photoacoustic monitoring system of claim 10, wherein the
memory stores instructions for determining whether the sensor
assembly is operating in an indicator dilution mode.
17. The photoacoustic monitoring system of claim 16, wherein the
memory stores instructions for increasing the repetition rate of
the emitted light if it is determined that the sensor assembly is
operating in the indicator dilution mode and that the sensor
assembly is applied to the tissue of the patient.
18. A method, comprising: emitting one or more wavelengths of light
from a light source of a sensor assembly into an interrogation
region of a patient; detecting an acoustic response to the emitted
light from the interrogation region of the patient with an acoustic
detector of the sensor assembly; processing a signal indicative of
whether the sensor assembly is positioned on a tissue of the
patient; and determining whether the sensor assembly is positioned
on the tissue of the patient based on the processed signal.
19. The method of claim 18, wherein the signal indicative of
whether the sensor assembly is positioned on the tissue comprises
the detected acoustic response.
20. The method of claim 18, comprising determining a current mode
of operation of the sensor assembly and triggering a response based
at least in part on the current mode of operation and based at
least in part on whether the sensor assembly is positioned on the
tissue of the patient.
21. The method of claim 20, comprising increasing the repetition
rate if it is determined that the current mode of operation is an
indicator dilution mode and the sensor assembly is positioned on
the tissue of the patient.
22. The method of claim 20, comprising increasing the power of the
light if it is determined that the current mode of operation is a
default photoacoustic mode and the sensor assembly is positioned on
the tissue of the patient.
Description
BACKGROUND
[0001] The present disclosure relates generally to medical devices
and, more particularly, to the use of photoacoustic spectroscopy in
patient monitoring.
[0002] This section is intended to introduce the reader to various
aspects of art that may be related to various aspects of the
present disclosure, which are described and/or claimed below. This
discussion is believed to be helpful in providing the reader with
background information to facilitate a better understanding of the
various aspects of the present disclosure. Accordingly, it should
be understood that these statements are to be read in this light,
and not as admissions of prior art.
[0003] In the field of medicine, medical practitioners often desire
to monitor certain physiological characteristics of their patients.
Accordingly, a wide variety of devices have been developed for
monitoring patient characteristics. Such devices provide doctors
and other healthcare personnel with the information they need to
provide healthcare for their patients. As a result, such monitoring
devices have become an indispensable part of modern medicine. For
example, in certain medical contexts, it may be desirable to
ascertain various localized physiological parameters, such as
parameters related to individual blood vessels or other discrete
components of the vascular system. Examples of such parameters may
include oxygen saturation, hemoglobin concentration, perfusion, and
so forth, for an individual blood vessel.
[0004] In one approach, measurement of such localized parameters is
achieved via photoacoustic (PA) spectroscopy. PA spectroscopy
utilizes light directed into a patient's tissue to generate
acoustic waves that may be detected and resolved to determine
localized physiological information of interest. In particular, the
light energy directed into the tissue may be provided at particular
wavelengths that correspond to the absorption profile of one or
more blood or tissue constituents of interest. In some systems, the
light is emitted as pulses (i.e., pulsed PA spectroscopy), though
in other systems the light may be emitted in a continuous manner
(i.e., continuous PA spectroscopy). The light absorbed by the
constituent of interest results in a proportionate increase in the
kinetic energy of the constituent (i.e., the constituent is
heated), which results in the generation of acoustic waves. The
acoustic waves may be detected and used to determine the amount of
light absorption, and thus the quantity of the constituent of
interest, in the illuminated region. For example, the detected
acoustic energy may be proportional to the optical absorption
coefficient of the blood or tissue constituent and the fluence of
light at the wavelength of interest at the localized region being
interrogated (e.g., a specific blood vessel).
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Advantages of the disclosed techniques may become apparent
upon reading the following detailed description and upon reference
to the drawings in which:
[0006] FIG. 1 is a block diagram of a patient monitor and a
photoacoustic sensor assembly in accordance with an embodiment;
[0007] FIG. 2 is a schematic illustrating a photoacoustic sensor
assembly having an angled light delivery system, in accordance with
an embodiment;
[0008] FIG. 3 is a schematic illustrating a photoacoustic sensor
assembly having an orthogonal light delivery system, in accordance
with an embodiment;
[0009] FIG. 4 is a schematic illustrating an ear clip style
photoacoustic sensor assembly, in accordance with an
embodiment;
[0010] FIG. 5 is a schematic of a photoacoustic sensor assembly
having an optical detector, in accordance with an embodiment;
[0011] FIG. 6 is a plot of an optical signal obtained as a sensor
assembly transitions from a position on a patient's skin to a
position off of the patient's skin, in accordance with an
embodiment;
[0012] FIG. 7 is a flow diagram of a method for determining an
appropriate response based on whether a photoacoustic sensor
assembly is applied to a patient's tissue, in accordance with an
embodiment; and
[0013] FIG. 8 is flow diagram of a method for determining an
appropriate response based on whether a photoacoustic sensor
assembly is applied to a patient's skin and a current mode of
operation, in accordance with an embodiment.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0014] One or more specific embodiments of the present techniques
will be described below. In an effort to provide a concise
description of these embodiments, not all features of an actual
implementation are described in the specification. It should be
appreciated that in the development of any such actual
implementation, as in any engineering or design project, numerous
implementation-specific decisions must be made to achieve the
developers' specific goals, such as compliance with system-related
and business-related constraints, which may vary from one
implementation to another. Moreover, it should be appreciated that
such a development effort might be complex and time consuming, but
would nevertheless be a routine undertaking of design, fabrication,
and manufacture for those of ordinary skill having the benefit of
this disclosure.
[0015] Presently disclosed embodiments of photoacoustic (PA)
sensors, systems, and methods are provided for the measurement of
various localized physiological parameters, such as parameters
related to individual blood vessels or other discrete components of
the vascular system. Examples of such parameters may include but
are not limited to oxygen saturation, hemoglobin concentration,
perfusion, cardiac output, and so forth, for an individual blood
vessel. In certain embodiments, the disclosed PA sensors may be
utilized as part of a PA spectroscopy system in which light is
directed into a patient's tissue to generate acoustic waves that
may be detected and resolved to determine the localized
physiological information of interest. In these embodiments, the
light energy directed into the tissue is provided at particular
wavelengths that correspond to the absorption profile of one or
more blood or tissue constituents of interest. In some embodiments,
the PA spectroscopy system may additionally or alternatively be
used to non-invasively measure indicator dilution, which may relate
to cardiac output and other hemodynamic parameters. For example,
for patients with an indicator solution injected into a vein, PA
monitoring techniques may be used to measure dilution of the
indicator in a downstream artery after mixing in the blood.
Disclosed embodiments may be utilized in PA spectroscopy systems in
which the light is emitted as pulses (i.e., pulsed PA
spectroscopy), as well as in systems in which the light is emitted
in a continuous manner (i.e., continuous PA spectroscopy).
Additionally, the disclosed embodiments may be used and/or combined
with the PA sensors and monitoring systems disclosed in U.S. patent
application Ser. No. 13/842,466 filed on Mar. 15, 2013, the
specification of which is incorporated by reference herein in its
entirety herein for all purposes.
[0016] Proper PA sensor placement is associated with improved
measurements. The use of high-intensity light sources, such as
lasers, in PA spectroscopy may also involve specialized training
and equipment, which may be complex and costly. Thus, certain
features of the disclosed embodiments may mitigate the effects of
high-intensity light sources in the PA sensor. For example, in
certain embodiments, the PA sensor includes a light emitter, such
as a laser diode, oriented at an angle such that the emitted light
is totally internally reflected when the sensor is exposed to air,
to reduce the intensity of the emitted light when the sensor is
away from a patient's skin. Accordingly, when the sensor is removed
from the tissue, little to no light is emitted. Additionally, in
certain embodiments, the associated system may monitor an acoustic
signal to detect unexpected changes or diminishment of the acoustic
signal. Unexpected change or diminishment of the acoustic signal
may indicate that the sensor is improperly placed on or removed
from the patient's tissue. In some embodiments, the PA sensor may
include an optical detector for detecting light emitted by the PA
light source. Characteristics of the detected optical signal may
indicate whether the sensor is appropriately placed on the
subject's skin. For example, the detected optical signal may
increase when the sensor is removed from the skin due to reflection
of the light off the skin toward the optical detector. Additionally
or alternatively, in certain embodiments, the PA sensor includes
one or more contact sensing elements, which may generate signals
indicative of whether the PA sensor is applied to the patient's
skin, for example.
[0017] As described in more detail below, the foregoing features
may facilitate the reduction in the emission of high-intensity
light when the PA sensor is not applied to the patient's skin. In
some embodiments, the PA spectroscopy system may alter
characteristics (e.g., power, repetition rate, pulse width) of the
emitted light based at least in part on whether the PA is applied
to the patient's skin, as discussed in more detail below.
[0018] With this understanding, FIG. 1 depicts a block diagram of a
photoacoustic spectroscopy system 8 in accordance with embodiments
of the present disclosure. The system 8 includes a photoacoustic
spectroscopy sensor 10 and a monitor 12. During operation, the
sensor 10 emits light at certain wavelengths into a patient's
tissue and detects acoustic shock waves (e.g., ultrasound waves)
generated in response to the emitted light. The monitor 12 is
capable of calculating physiological characteristics based on
signals received from the sensor 10 that correspond to the detected
acoustic shock waves. The monitor 12 may include a display 14
and/or a speaker 16, which may be used to convey information about
the calculated physiological characteristics to a user. The monitor
12 may be configured to receive user inputs via control inputs 17.
The sensor 10 may be communicatively coupled to the monitor 12 via
a cable or, in some embodiments, via a wireless communication
link.
[0019] In one embodiment, the sensor 10 may include a light source
18 and an acoustic detector 20, such as an ultrasound transducer.
The light source 18 may emit light as pulses or in a continuous
manner. Further, in certain embodiments, the light source 18 may be
associated with one or more optical fibers for conveying light from
one or more light generating components to the tissue site. In some
embodiments, the sensor 10 may also include an optical detector 22
that may be a photodetector, such as a silicon photodiode package,
selected to receive light in the range emitted from the light
source 18. In the present context, the optical detector 22 may be
referred to as a detector, a photodetector, a detector device, a
detector assembly or a detector component. Further, the optical
detector 22 and light source 18 may be referred to as optical
components or devices. In some embodiments, the optical detector 22
is configured to receive light emitted by the light source 18,
although in other embodiments, the sensor 10 may include an
additional light source for use with the optical detector 22.
[0020] The sensor 10 may include the light source 18 and the
acoustic detector 20 that may be of any suitable type. For example,
in one embodiment, the light source 18 may include one, two, or
more light emitting components (such as light emitting diodes or
LEDs) adapted to transmit light at one or more specified
wavelengths. In certain embodiments, the light source 18 may
include a laser diode or a vertical cavity surface emitting laser
(VCSEL). The laser diode may be a tunable laser, such that a single
diode may be tuned to various wavelengths corresponding to a number
of different absorbers of interest in the tissue and blood. That
is, the light may be any suitable wavelength or wavelengths (such
as a wavelength between about 500 nm to about 1000 nm or between
about 600 nm to about 900 nm) that is absorbed by a constituent of
interest in the blood or tissue. For example, wavelengths between
about 500 nm to about 600 nm, corresponding with green visible
light, may be absorbed by deoxyhemoglobin and oxyhemoglobin. In
other embodiments, red wavelengths (e.g., about 600 nm to about 700
nm) and infrared or near infrared wavelengths (e.g., about 800 nm
to about 1000 nm) may be used. In one embodiment, the selected
wavelengths of light may penetrate into the tissue of a patient 24
up to approximately 1 mm to approximately 3 cm. In certain
embodiments, the selected wavelengths may penetrate through bone
(e.g., the rib cage) of the patient 24. In disclosed embodiments
that include the light source 18, it should be understood that the
light source 18 may be coupled to an optical fiber.
[0021] To increase the precision of the measurements, the emitted
light may be focused on an internal region of interest by
modulating the intensity and/or phase of the illuminating light.
Accordingly, an acousto-optic modulator (AOM) 25 may modulate the
intensity of the emitted light, for example, by using linear
frequency modulation (LFM) techniques. The emitted light may be
intensity modulated by the AOM 25 or by changes in the driving
current of the LED emitting the light. The intensity modulation may
result in any suitable frequency, such as from 1 MHz to 10 MHz or
more. Accordingly, in one embodiment, the light source 18 may emit
LFM chirps at a frequency sweep range approximately from 1 MHz to 5
MHz. In another embodiment, the frequency sweep range may be of
approximately 0.5 MHz to 10 MHz. The frequency of the emitted light
may be increasing with time during the duration of the chirp. In
certain embodiments, the chirp may last approximately 0.1 second or
less and have an associated energy of a 10 mJ or less, such as
between 1 .mu.J to 2 mJ, 1-5 mJ, 1-10 mJ. In such an embodiment,
the limited duration of the light may prevent heating of the tissue
while still emitting light of sufficient energy into the region of
interest to generate the desired acoustic waves when absorbed by
the constituent of interest.
[0022] In disclosed embodiments, the acoustic waves may be detected
by the acoustic detector 20, which may include an ultrasound
transducer or transducer array. In one example, the acoustic
detector 20 may be one or more ultrasound transducers, such as a
focused ultrasound transducer, suitable for detecting ultrasound
waves emanating from the tissue in response to the emitted light
and for generating a respective optical or electrical signal in
response to the ultrasound waves. For example, the acoustic
detector 20 may be suitable for measuring the frequency and/or
amplitude of the acoustic waves, the shape of the acoustic waves,
and/or the time delay associated with the acoustic waves with
respect to the light emission that generated the respective waves.
In one embodiment an acoustic detector 20 may be an ultrasound
transducer employing piezoelectric or capacitive elements to
generate an electrical signal in response to acoustic energy
emanating from the tissue of the patient 24, i.e., the transducer
converts the acoustic energy into an electrical signal. The
acoustic detector 20 may be made, for example, of piezoelectric
materials such as lead zirconate titanate (PZT), polyvinylidene
fluoride (PVDF), and so forth.
[0023] In some embodiments, the system 8 may also include any
number or combination of additional medical sensors for providing
information related to patient parameters that may be used in
conjunction with the PA spectroscopy sensor 10. For example,
suitable additional medical sensors may include sensors for
determining blood pressure, blood constituents, respiration rate,
respiration effort, heart rate, patient temperature, cardiac
output, and so forth. Such information may be used, for example, to
determine if the patient is in shock or has an infection.
Additionally, the system 8 may also include one or more contact
sensing elements 26 (e.g., sensing elements) configured to generate
signals or to provide information related to whether the sensor 10
is applied to the patient's skin. Such sensing elements 26 may be
included within or may be coupled to the sensor 10, or may
otherwise be incorporated into or be in communication with the
system 8. As described in detail below, the sensing elements 26 may
include an impedance sensor or a temperature sensor, for
example.
[0024] The sensor 10 may include a memory or other data encoding
component, depicted in FIG. 1 as an encoder 28. For example, the
encoder 28 may be a solid state memory, a resistor, or combination
of resistors and/or memory components that may be read or decoded
by the monitor 12, such as via reader/decoder 30, to provide the
monitor 12 with information about the attached sensor 10. For
example, the encoder 28 may encode information about the sensor 10
or its components (such as information about the light source 18
and/or the acoustic detector 20). Such encoded information may
include information about the configuration or location of
photoacoustic sensor 10, information about the type of lights
source(s) 18 present on the sensor 10, information about the
wavelengths, light wave frequencies, chirp durations, and/or light
wave energies which the light source(s) 18 are capable of emitting
and the properties and/or detection range of the optical detector
22, information about the nature of the acoustic detector 20, and
so forth. In certain embodiments, the information also includes a
reference LFM chirp that was used to generate the actual LFM
emitted light. This information may allow the monitor 12 to select
appropriate algorithms and/or calibration coefficients for
calculating the patient's physiological characteristics, such as
the amount or concentration of a constituent of interest in a
localized region, such as a blood vessel.
[0025] In one implementation, signals from the acoustic detector 20
(and decoded data from the encoder 28, if present) and the optical
detector 22 may be transmitted to the monitor 12. The monitor 12
may include data processing circuitry (such as one or more
processors 32, application specific integrated circuits (ASICS), or
so forth) coupled to an internal bus 34. Also connected to the bus
34 may be a RAM memory 36, a ROM memory 38, a speaker 16 and/or a
display 14. In one embodiment, a time processing unit (TPU) 40 may
provide timing control signals to light drive circuitry 42, which
controls operation of the light source 18, such as to control when,
for how long, and/or how frequently the light source 18 is
activated, and if multiple light sources are used, the multiplexed
timing for the different light sources.
[0026] The TPU 40 may also control or contribute to operation of
the acoustic detector 20 and/or the optical detector 22 such that
timing information for data acquired using the acoustic detector 20
and/or the optical detector 22 may be obtained. Such timing
information may be used in interpreting the acoustic wave data
and/or in generating physiological information of interest from
such acoustic data. For example, the timing of the acoustic data
acquired using the acoustic detector 20 may be associated with the
light emission profile of the light source 18 during data
acquisition. Likewise, in one embodiment, data acquisition by the
acoustic detector 20 may be gated, such as via a switching circuit
44, to account for differing aspects of light emission. For
example, operation of the switching circuit 44 may allow for
separate or discrete acquisition of data that corresponds to
different respective wavelengths of light emitted at different
times. Similarly, the data acquired from the optical detector 22
may be gated via the switched circuit 44.
[0027] The received signal from the acoustic detector 20 and/or the
optical detector 22 may be amplified (such as via amplifier 46),
may be filtered (such as via filter 48), and/or may be digitized if
initially analog (such as via an analog-to-digital converter 50).
The digital data may be provided directly to the processor 32, may
be stored in the RAM 36, and/or may be stored in a queued serial
module (QSM) 52 prior to being downloaded to RAM 36 as QSM 52 fills
up. In one embodiment, there may be separate, parallel paths for
separate amplifiers, filters, and/or A/D converters provided for
different respective light wavelengths or spectra used to generate
the acoustic data. Further, while the disclosed block diagram shows
the signal from the optical detector 22 and the acoustic detector
20 being supplied to the same path (e.g., a path that may include a
switch 44, amplifier 46, filter 48, A/D converter 50, and/or a QSM
52), it should be understood that these signals may be received and
processed on separate paths or separate channels.
[0028] The data processing circuitry, such as processor 32, may
derive one or more physiological characteristics based on data
generated by the sensor 10. For example, based at least in part
upon data received from the acoustic detector 20, the processor 32
or other suitable circuitry may calculate the amount or
concentration of a constituent of interest in a localized region of
tissue or blood using various algorithms. In one embodiment, the
processor 32 may calculate one or more of cardiac output, total
blood volume, extravascular lung water, intrathoracic blood volume,
systemic and pulmonary blood flow, and/or macro and microvascular
blood flow from signals obtained from a signal sensor 10. In
certain embodiments, these algorithms may use coefficients, which
may be empirically determined, that relate the detected acoustic
waves generated in response to emitted light waves at a particular
wavelength or wavelengths to a given concentration or quantity of a
constituent of interest within a localized region.
[0029] In one embodiment, the processor 32 may access and execute
coded instructions, such as for implementing the algorithms
discussed herein, from one or more storage components of the
monitor 12, such as the RAM 36, the ROM 38, and/or a mass storage
54. Additionally, the RAM 36, ROM 38, and/or the mass storage 54
may serve as data repositories for information such as templates
for LFM reference chirps, coefficient curves, and so forth. For
example, code encoding executable algorithms may be stored in the
ROM 38 or mass storage device 54 (such as a magnetic or solid state
hard drive or memory or an optical disk or memory) and accessed and
operated according to processor 32 instructions using stored data.
Such algorithms, when executed and provided with data from the
sensor 10, may calculate one or more physiological characteristics
as discussed herein (such as the concentration or amount of a
constituent of interest). Once calculated, the physiological
characteristics may be displayed on the display 14 for a caregiver
to monitor or review.
[0030] With the foregoing system discussion in mind, light emitted
by the light source 18 of the sensor 10 may be directed into a
patient's tissue to generate acoustic signals in proportion to the
amount of an absorber (e.g., a constituent of interest, such as a
saline indicator) in a targeted localized region. However, as noted
above, one problem that may arise in photoacoustic spectroscopy is
that the light source 18 may emit a high-intensity light, and it
may be desirable to control the emission of the high-intensity
light for improved measurements, for example. Accordingly, certain
embodiments of the present disclosure relate to PA spectroscopy
systems 8 configured so that the high-intensity light emitted by
the light source 18 is totally internally reflected within the
sensor 10 if the sensor 10 is surrounded by air (e.g., not on the
patient's skin), or so that the light source 18 emits light only if
the sensor 10 is positioned on the patient's skin. Additionally,
certain embodiments of the present disclosure relate to PA
spectroscopy systems 8 having various configurations and/or
features that enable the system 8 to determine whether the sensor
10 is positioned on the patient's skin. The system 8 may, in turn,
be configured to adjust or control emission of light from the light
source 18 based at least in part on whether the sensor is
positioned on the patient's skin. Examples of suitable
configurations of the sensor 10 are discussed in more detail
below.
[0031] FIG. 2 illustrates an embodiment of a PA sensor assembly 90
including an optically transparent and ultrasound coupling spacer
92, an acoustic detector 94, a laser diode assembly 96 (e.g., an
angled laser diode assembly), and a housing 98. The angled laser
diode assembly 96 is configured to emit light into the spacer 92
for patient monitoring. In certain embodiments, the spacer 92 is a
Rexolite prism. Rexolite is utilized as the spacer 92 in some
embodiments because of its low ultrasound attenuation, high light
transmission, and its ability to be machined to the prism shape,
which facilitates tuning of the direction of ultrasound propagation
during operation of the sensor assembly 90. However, in other
embodiments, any desired spacer 92 having any desired features may
be employed, not limited to Rexolite, depending on
implementation-specific considerations. For instance, in some
embodiments, the spacer 92 may be any material having a low
ultrasound impedance (i.e., an ultrasound impedance approximately
equal or close to the ultrasound impedance of the tissue of the
patient) and high light transmission. For example, in one
embodiment, the ultrasound impedance of the spacer 92 may be
approximately 1.5-1.6 MRayls. Additionally, the sensor assembly 90
may include any suitable acoustic detector 94.
[0032] As shown in FIG. 2, in certain embodiments, the angled laser
diode assembly 96 may be positioned so that light is delivered at a
non-orthogonal angle relative to the patient's tissue when the
sensor 10 is in operation. The spacer 92 includes an angled face or
portion 100 that accommodates the angled laser diode assembly 96.
In some embodiments, the angled laser diode assembly 96 may
transmit light into an angled optical fiber 102 disposed within an
angled optical channel 104, although in other embodiments, the
angled laser diode assembly 96 emits light into the spacer 92
without the use of the angled optical fiber 102 or the angled
optical channel 104. During operation, the sensor assembly 90 is
positioned with respect to the patient such that a bottom surface
110 of the spacer 92 is in contact with the surface of the
patient's tissue. Once the light is emitted and transmitted into
the patient, the returning acoustic signal travels through the
spacer 92 to the acoustic detector 94.
[0033] In some embodiments, by providing the angled laser diode
assembly 96 the likelihood that light will escape the spacer 92
when the light delivery device is removed from the surface of the
patient's skin (i.e., surface 110 is no longer in contact with the
surface of the patient's skin) may be significantly reduced or
eliminated. This feature may offer advantages by decreasing the
intensity of the light that escapes the sensor assembly 90 and/or
by decreasing the likelihood that emitted light reaches the patient
or others in the surrounding environment when the assembly is not
positioned for use on tissue (e.g., when the sensor assembly is
carried or lifted for repositioning by an operator).
[0034] More specifically, an angle 112 between the emitted light
and a side surface 114 of the spacer 92 may be selected such that a
light delivery angle is larger than the critical angle (i.e., the
angle of incidence above which total internal reflection occurs)
for the spacer 92 to air interface. The critical angle depends on
the refractive indices (n) of the materials, and may generally be
determined by the following equation:
.theta..sub.1=arcsin(n.sub.2/n.sub.1)
[0035] For example, in embodiments in which Rexolite is used as the
spacer 92, the angle 112 may be selected such that the light
delivery angle is larger than approximately 39 degrees, which is
the critical angle for the Rexolite (n.sub.1=1.57) to air
(n.sub.2=1.0) interface. In embodiments in which the angle 112 is
in this manner, the emitted light will be totally internally
reflected when the sensor assembly 90 is removed from the surface
of the patient's tissue. Therefore, the emitted light will remain
reflected within the sensor assembly, and will not emit into the
surrounding environment, thereby reducing or eliminating the
likelihood that an operator or others in the surrounding
environment are exposed to the emitted light when the sensor
assembly is removed from the patient. When the sensor assembly 90
is in contact with the patient's skin, the light delivery angle 112
is less than the critical angle due to the change of index of
refraction at the spacer 92 to tissue (n.sub.3=1.4) interface, and
therefore the light is emitted into the patient tissue as desired
for the PA response. An additional consideration is the critical
angle between the spacer 92 and the patient's skin, and any angle
greater than the critical angle will not pass through to the
patient's skin. The angle 112 at which the light is emitted from
the angled laser diode assembly 96 is preferably controlled between
a first critical angle (e.g., the critical angle for the spacer to
air interface) and a second critical angle (e.g., the critical
angle for the spacer to tissue interface). In certain embodiments,
the angle 112 is preferably controlled to approach or to be near
the second critical angle so that a higher percentage of light
enters the patient's tissue and a lower percentage of light is
internally reflected when the spacer 92 is positioned on the
patient's tissue. By way of example, if the spacer 92 is formed
from Rexolite, the critical angle for the Rexolite to tissue
interface is approximately 63 degrees. In some embodiments, the
angle 112 of incidence is preferably controlled between
approximately 39 and 63 degrees. In certain embodiments, the angle
112 is preferably controlled to approach 63 degrees (e.g., be
approximately 63 degrees, between about 60 and 63 degrees, between
about 55 and 63 degrees, between about 50 and 63 degrees, etc.) so
that a higher percentage of light enters the patient's tissue and a
lower percentage of light is reflected when the Rexolite spacer is
positioned on the patient's tissue. Thus, the spacer 92 and the
angled laser diode assembly 96 can be configured to facilitate
total internal reflection of the emitted light when the sensor
assembly 90 is not applied to the patient, but to enable
transmission of the emitted light to the patient tissue when the
sensor assembly 90 is applied to the patient's skin. Such a
configuration may prevent emission of high intensity light unless
the surface 110 is in contact with the patient's skin. Additionally
or alternatively, the spacer 92 may be configured to allow the
laser light to expand as it propagates through the spacer lowering
the light density. In operation, after the light is transmitted to
the patient's tissue, the PA signal may be received on the bottom
surface 110 of spacer 92 and transmitted through the spacer 92 to
acoustic detector 94.
[0036] The angled laser diode assembly 96 may be connected by a
cable 116 to a power source and/or medical device. In other
implementations, any of the disclosed embodiments provided herein
may also be configured as wireless sensors. In certain embodiments,
the sensor assembly 90 may include the housing 98 that fully or
partially surrounds the spacer 92, the angled laser diode assembly
96, and/or the acoustic detector 94, for example. The housing 98
may be generally configured to protect the sensor assembly 90, and
the housing 98 may take any suitable form and may be formed from
any suitable materials. For example, in some embodiments, the
housing 98 may be formed from plastic or metal materials, or any
combination thereof. In some embodiments, the housing 98 may be
opaque or may have a colored interior surface (e.g., dyed, painted,
pigmented, etc.) configured to absorb light that escapes the spacer
92. The housing 98 may thus reduce the amount of light that escapes
the sensor assembly 90. In some embodiments, an air gap 118 is
provided between the spacer 92 and the housing 98 to reduce or
prevent any acoustic signals from being generated at the housing
surface where the light is absorbed. Additionally, the air gap 118
may provide a resistance so that any acoustic signal that is
generated at the housing surface is not able to transmit through
the air gap to the acoustic detector 94.
[0037] As set forth above, FIG. 2 depicts an embodiment of the
sensor assembly 90 that is configured to control the emission of
light by providing the angled laser diode assembly 96 to facilitate
total internal reflection of the emitted light when the sensor
assembly 90 is not applied to the patient's skin. Additionally or
alternatively, the PA spectroscopy system 8 may include other
light-controlling features. For example, the sensor assembly 90 may
include or may be coupled to one or more sensing elements 26 that
generate signals indicative of whether the sensor assembly 90 is
positioned on the patient's skin, as discussed in more detail
below. The system 8 may, in turn, be configured to operate and/or
control the light source 18 based at least in part on whether the
sensor assembly is determined to be on the patient's skin.
[0038] The sensing elements 26 may be used with any suitable sensor
assembly, such as the sensor assembly 90, having the angled laser
diode 96. Additionally, as shown in FIG. 3, the sensing elements 26
may be used in a sensor assembly 120 in which a laser diode
assembly 122 is positioned so that the emitted light is
approximately orthogonal (e.g., perpendicular, 90 degrees, etc.) to
the patient's tissue. In some instances, during operation, this
orthogonality may result in a reduced quantity of background noise
as compared to non-orthogonal designs, as more light is absorbed
into the skin and less is reflected, and the higher light density
associated with such configurations may result in a stronger
photoacoustic signal Additionally, the orthogonally emitted light
has a higher light power density, resulting in a stronger acoustic
signal. Additionally, the embodiment of FIG. 3 may offer certain
advantages, such as enabling more control over a spot size of the
emitted light that reaches the patient and accommodating a variety
of sizes of acoustic detectors 124. The emitted light and acoustic
waves may pass through a spacer 126.
[0039] With reference to FIG. 3, the sensing elements 26 may be
coupled to the sensor assembly 120 in any suitable manner and may
be disposed in any suitable location, such as on a bottom surface
128 of the sensor assembly 120. Although the sensor assembly 120 is
shown without a housing, it should be understood that a housing,
such as housing 98 in FIG. 2, may surround the spacer 126 and that
an air gap, such as air gap 118, may be provided between the spacer
126 and the housing. In some embodiments, the sensing elements 26
may be recessed within the spacer 126. Such a configuration may
protect the sensing elements 26 during use and/or may allow the
sensing elements 26 to be flush (e.g., level) with the bottom
surface 128 of the spacer 126, which in turn may aid in placing the
sensor assembly 120 properly on the patient's tissue. Such a
configuration may also enable the sensing elements 26 to accurately
detect when the sensor assembly 120 peels away (e.g., lifts off,
separates from, etc.) the patient's skin. In some embodiments, the
sensing elements 26 may be coupled to a flex circuit or a plate
(e.g., a metal plate) that is configured to be coupled to (e.g.,
removably attached or fixedly attached) to the sensor assembly 120
(e.g., the bottom surface 128 of the sensor assembly 120. In
certain embodiments, the sensing elements 26 may be placed outside
of a pathway 130, through which the emitted light and/or acoustic
waves travel through the spacer 126. The sensing elements 26 may be
positioned outside of the pathway 130 or in any position in which
the sensing elements 26 do not interfere with the emitted light
and/or the acoustic waves. Furthermore, any suitable sensing
elements 26 may be utilized. For example, any sensing elements 26
configured to generate signals indicative of whether the sensor
assembly 120 is off, on, and/or near the patient's skin may be used
in the presently disclosed system 8. Thus, as set forth below, the
sensing elements 26 may include impedance sensors, temperature
sensors, pressure sensors, and the like. In some embodiments, the
sensing elements 26 may comprise an optical detector configured to
generate an optical signal that may indicate whether the sensor
assembly 120 is positioned on the patient's tissue. In certain
embodiments, the sensing elements 26 may broadly comprise the
acoustic detector 124 detector configured to generate the acoustic
signal that may indicate whether the sensor assembly 120 is
positioned on the patient's tissue (e.g., that may be processed by
the monitor 12 to determine whether sensor assembly 120 is
positioned on the patient's tissue).
[0040] In some embodiments, the sensing elements 26 may include one
or more electrical contacts or impedance sensors. In certain
embodiments having such impedance sensors, two or more electrodes
may be positioned on a bottom surface of the sensor assembly 120.
The system 8 may be configured to measure a voltage corresponding
to a current flowing between the electrodes, the measured voltage
being proportional to the impedance between the electrodes. Since
the impedance of the air surrounding the sensor assembly 120 is
higher than the impedance of the surface of the patient's skin, the
measured voltage will be relatively low when the electrodes are
placed firmly against the patient's skin, and the voltage will be
relatively high when both electrodes are exposed to the air (e.g.,
when the electrical path between the electrodes is through the
air). Thus, the measured voltage may be compared to a certain
threshold voltage to determine whether the electrodes of the
sensing elements 26 (and thus the sensor assembly 120) are applied
to the patient's skin. Any of a variety of suitable types of
sensing elements 26, such as thermistors or temperature sensors,
may be utilized to detect whether the sensor assembly 120 is
positioned on the patients skin via various sensed parameters
(e.g., temperature). Additionally, various combinations of one or
more different types of sensing elements 26 may be used with the
sensor assembly 120. Thus, in some embodiments, two or more
different types of sensing elements 26 may be provided and/or
coupled to the sensor assembly 120 to provide signals indicative of
whether the sensor assembly 120 is positioned one the patient's
skin.
[0041] The sensing elements 26 may be spaced or located on the PA
sensor assembly 120 in accordance with the intended orientation of
the PA sensor on the patient. PA sensor assemblies, such as the
sensor assembly 120, may be placed in any suitable location on the
patient 24. As shown in FIG. 4, in certain embodiments, the sensor
assembly 120 may be placed on or near the patient's temple. In some
such embodiments, the sensor assembly 120 may include a support
structure 140 (e.g., clip, hook, arm, etc.) configured to couple
the sensor assembly 120 to the patient's ear. The sensor assembly
120 may be relatively heavy compared to other types of medical
sensors (e.g., pulse oximeters), and thus the sensor assembly 120
may have a tendency to separate from the patient's skin. In some
configurations, the sensor assembly 120 may have a tendency to
separate from the patient's skin at the corners or edges of the
sensor assembly 120. For example, when the patient is standing or
in an upright position, a top edge 142 (e.g., first edge) may have
a tendency to separate from the patient's skin, and thus, it may be
desirable to provide sensing elements 26a, 26b near or adjacent to
the top edge 142 so that separation of the sensor assembly 120 from
the patient's skin is detected as early as possible during should
the sensor assembly 120 separate or dislodge from the patient's
skin. In certain embodiments, the sensor assembly 120 may be
configured to be applied to a supine patient, and thus, sensing
elements 26b, 26c may desirably be positioned near or adjacent to a
front edge 144 (e.g., second edge), as the sensor may be more
likely to separate at the front edge 144 first when the patient is
lying down. In some embodiments, the sensing elements 26a, 26b,
26c, 26d may be positioned at or near each corner of a generally
rectangular bottom surface 128 of the sensor assembly 120. The
sensing elements 26 may also have any shape or form, and thus, the
sensing elements 26 may be arranged or formed into strips or lines
that are positioned adjacent to one or more edges of the sensor
assembly 120, for example.
[0042] Additionally or alternatively, the system 8 may be
configured to determine whether the sensor assembly 120 is
positioned on the patient's skin through other techniques, such as
an optical technique. As shown in FIG. 5, a sensor assembly 150 may
include an optical detector 156 as well as a light source 152 and
an acoustic detector 154. When the sensor assembly 150 is applied
to a patient, the light source 152 directs light 158 toward a
target blood vessel 160, in a manner as described above. A portion
of the light 158 is absorbed in the blood vessel 160 to generate
acoustic waves 162. Another portion 164 scatters toward the optical
detector 156 and is detected. The acoustic detector 154 and the
optical detector 156 generate signals representative of the
detected acoustic waves 162 and the detected light 164,
respectively.
[0043] In accordance with certain embodiments, the signals
generated by the optical detector 156 may be utilized by the system
8 to determine whether the sensor assembly 150 is positioned on the
patient's skin. Any suitable processing method for determining
signal quality may be employed to assess the quality of the
received signal and to determine if the signal meets a certain
threshold quality. For example, the signal may be qualified based
on a pulse qualification. Through such techniques, the signal may
be evaluated to identify a pulse of blood due to a heart beat, and
the presence of such a pulse may indicate that the sensor assembly
150 is on the skin. In some embodiments, the signal may be
qualified based on a ratio of ratios of the signal. Through such
techniques, the signal may be evaluated to determine an oxygen
saturation measurement, which may indicate that the sensor is
properly placed over an artery on the skin and may be used by the
system 8 to determine that the sensor assembly 150 is positioned on
the patient's skin. Signal quality assessments and signal
qualification may be performed via any suitable technique, such as
the techniques provided in U.S. Pat. No. 7,209,774, the
specification of which is incorporated by reference herein in its
entirety herein for all purposes.
[0044] As set forth above, the system 8 may be configured to
determine whether the sensor assembly 150 is applied to the
patient's skin based at least in part on the signal qualification.
For example, the system 8 may determine that the sensor assembly
150 is applied to the patient's skin if various metrics related to
the received signal or characteristics of the received signal meet
certain minimum thresholds or is otherwise determined to be
qualified. In some embodiments, the system 8 may determine and/or
qualify or evaluate various metrics that quantify one or more
aspects of the signal. The system 8 may determine and/or evaluate
various metrics such as a heart rate, an average pulse period, an
amplitude of the signal, a ratio of ratios, a pulse shape (e.g.,
skew of the signal or skew of the derivative), and/or a frequency
of the signal. For example, the system 8 may compare a calculated
heart rate metric to certain thresholds, such as a minimum heart
rate of 35, 40, 50, or more beats per minute (bpm). Thus, by way of
example, if system 8 calculates a heart rate lower than 50 bpm,
then the system 8 may determine that the sensor assembly 150 is not
applied to the patient's tissue. The threshold may be set at a
manufacturing stage, may be adaptively set prior to or during a
monitoring session for each patient based on historical data,
and/or may be set or adjusted by a user based on user preferences
and/or patient characteristics. In certain embodiments, the system
8 may determine that the sensor assembly 150 has become dislodged
from the skin and/or is not positioned on the patient's skin if the
metric (e.g., the heart rate metric, the amplitude, etc.) changes
by more than 1%, 5%, 10%, 15%, 20% or more over a period of time
(e.g., 1, 2, 3, 4, 5, or more seconds) during a monitoring session.
Again, such tolerances or metrics may be set at a manufacturing
stage, may be adaptively set prior to or during a monitoring
session for each patient based on historical data, and/or may be
set or adjusted by a user based on user preferences and/or patient
characteristics, for example. The determination of whether the
signal is qualified may trigger certain actions, as described in
more detail below.
[0045] The above-described technique may be used with light sources
152 that emit a single wavelength of light (e.g., light in a single
spectrum, such as light in the IR spectrum) or that emit two or
more wavelengths of light (e.g., light in the IR and the red
spectrum). If the light source 152 is configured to emit two
wavelengths of light, the system 8 may additionally be configured
to generate plethysmography (PPG) signals and to determine various
types of physiological information (e.g., oxygen saturation, etc.).
The PPG signal may also be qualified via the techniques described
above to determine whether the sensor assembly 150 is applied to
the patient's skin.
[0046] However, as shown in FIG. 4, in certain embodiments, the
sensor assembly 120 may be placed on a temple of the patient or
over a large artery. Such locations are typically not well-suited
for pulse oximetry or for determining oxygen saturation based on
the PPG signal. Furthermore, certain light sources 152 may be bulky
and or heavy. Thus, in some embodiments, it may be desirable for
the light source 152 to emit only one wavelength of light (e.g.,
light in a single spectrum, such as the IR spectrum), and the
system 8 may be configured to qualify the signal generated by the
optical detector using only one wavelength of light via the
techniques described above and/or the techniques described below.
Such a configuration may use less hardware and/or may reduce
processing steps, while enabling qualification of the optical
signal for determining whether the sensor assembly 150 is applied
to the patient's skin.
[0047] In some embodiments, the optical detector 156 may
additionally or alternatively be configured to monitor an optical
reflectance of light from the patient's skin. In particular, the
optical detector 156 may receive the light reflected from the
patient's tissue, and the system 8 may be configured to monitor a
direct current (DC) component of the optical signal to determine
whether the sensor assembly is positioned near the patient's skin.
FIG. 6 is a plot of an example of a DC component 170 of the optical
signal as the sensor (e.g., photoacoustic sensor assembly 150
having an optical detector 156) transitions from a position on a
patient's skin to a position off of the patient's skin, in
accordance with an embodiment. The DC component 170 of the optical
signal is a non-pulsatile component of the signal and is the result
of light absorption by nonpulsatile tissue, such as fat, bone,
muscle, and skin. As shown, the sensor assembly 150 is initially on
the patient's skin at a first time 172. The DC component 170 is
expected to increase sharply as shown at a second time 174 when the
sensor assembly 150 initially peels away (e.g., lifts off,
separates from, etc.) and then to decrease as shown at a third time
176 as the sensor assembly 150 moves away from the patient's skin.
The changes in the DC component 170 of the optical signal are due,
at least in part, to the increased in light reflected from the
surface of the skin to the optical detector 156 when the sensor
assembly 150 is a first distance (e.g., a short distance) away from
the skin, as the sensor assembly 150 moves to a second distance
(e.g., a far distance, greater than the first distance) less
emitted light reflects off the skin and/or less light reflected
from the skin reaches the optical detector 156. Thus, the monitor
12 may be configured to monitor absolute values of the DC component
170 and/or to monitor relative changes of the DC component 170 over
time to determine whether the sensor assembly 150 is applied to the
patient's tissue. If the DC component 170 exceeds a certain
threshold and/or if a certain or expected pattern (e.g., a sharp
increase in the signal followed by a decrease in the signal) in the
DC component 170 is detected, the system 8 may determine that the
sensor assembly is not positioned on the patient's skin. In some
embodiments, if the amplitude of the DC component decreases by more
than about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more percent within 1,
2, 3, 4, 5, 6, 7, 8, 9, 10, or more seconds, the system 8 may
determine that the sensor assembly 150 is not positioned on the
patient's skin. In certain embodiments, if the amplitude of the DC
component changes by more than about 5 or more than about 10 times
its value within about 1, 2, or 3 seconds, the system 8 may be
configured to determine that the sensor assembly 150 is not
positioned on the patient's skin. Again, such tolerances or
thresholds may be set at a manufacturing stage, may be adaptively
set prior to or during a monitoring session for each patient based
on historical data, and/or may be set or adjusted by a user based
user preferences and/or patient characteristics, for example.
[0048] While the illustrated embodiment of FIG. 5 shows a sensor
arrangement in which the acoustic detector 154 is between the light
source 152 and the optical detector 156, it should be understood
that other arrangements are contemplated. For example, the optical
detector 156 may be positioned adjacent to the acoustic detector
154 but not along an axis connecting the light source 152 and the
acoustic detector 154. In certain embodiments, the optical detector
156 and the acoustic detector 154 are directly next to or adjacent
to one another. In one example of such an arrangement, the housings
or support structures for these elements may contact one another.
In this manner, the negative correlation between their signals may
be enhanced. In another embodiment, the optical detector 156 and
the acoustic detector 154 are spaced apart from one another.
Additionally, while only one light source 152 is illustrated in
FIG. 5, it should be understood than any suitable number of light
sources 152 may be used in the sensor assembly 150. The geometry of
the arrangement of the optical and acoustic components on the
sensor assembly 150 may influence the calibration of the sensor and
may be provided as an input to certain algorithms. Accordingly, in
one embodiment, sensor geometry information as well as other sensor
identification information and/or calibration information may be
stored in the encoder 28.
[0049] Additionally or alternatively, the system 8 may be
configured to determine whether the sensor assembly 150 (or sensor
assembly 120, for example) is positioned on the patient's skin
through other techniques, such as acoustic techniques. For example,
in certain embodiments, sensor assembly 150 may be configured to
monitor an intensity of the acoustic signal 162 and/or changes in
the acoustic signal 162 and to determine whether the acoustic
signal 162 indicates that the sensor assembly 150 is not on the
patient's skin. In certain embodiments, the system 8 may be
configured to determine whether sensor 10 is applied to the
patient's tissue based on the intensity of the acoustic signal 162
or changes in the acoustic signal 162 over a period of time. The
system 8 may be configured to monitor a quality of the acoustic
signal 162 based on various parameters or features of the received
signal (e.g., amplitude, shape, etc.). For example, sudden
diminishment (e.g., decrease in amplitude) of the acoustic signal
162 may indicate that the sensor assembly 150 has become dislodged
and/or is not positioned on the patient's skin. In certain
embodiments, if the acoustic signal 162 decreases by more than
about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more percent within 1, 2,
3, 4, 5, 6, 7, 8, 9, 10, or more seconds, the system 8 may
determine that the sensor assembly 150 is not positioned on the
patient's skin or has dislodged (e.g., peeled off) the patient's
skin. As noted above, such tolerances or thresholds may be set at a
manufacturing stage, may be adaptively set prior to or during a
monitoring session for each patient based on received signals or
patient data, and/or may be set or adjusted by a user based on user
preferences and/or patient characteristics, for example.
[0050] The above-described embodiments relate generally to
photoacoustic sensor systems 8 that are configured to determine
whether the sensor 10 (or sensor assembly 90, 120, 150, for
example) is applied to the patient's skin. The system 8 may also be
configured to trigger an appropriate response based at least in
part on whether it is determined that the sensor 10 is applied to
the patient's skin. For example, the system 8 may determine whether
the sensor 10 is applied to the skin using one or more of the above
techniques, and the information may be provided to a controller
that is configured to control the operation of the light source
18.
[0051] FIG. 7 is a flow diagram of one technique for operating the
sensor 10 and/or controlling the light source 18 in accordance with
the present disclosure. As shown, at step 180, the signal
indicative of whether the sensor 10 is positioned on the patient's
tissue is received (e.g., at the microprocessor 32). As indicated
above, the signal may be a voltage or current signal generated by
the sensing elements 26, an optical signal generated by the optical
detector 22, or an acoustic signal generated by the acoustic
detector 20, for example. At step 182, the system 8 processes the
signal to determine whether the sensor 10 is positioned on the
patient's skin, based on the techniques described above for each
type of signal. As shown in steps 184 and 186, certain actions are
triggered based on whether it is determined that the sensor 10 is
positioned on the patient's skin. For example, if it is determined
that the sensor is not on the patient's skin, the system 8 may
disable the light source 18 as shown in step 184. In certain
embodiments, the system 8 may additionally or alternatively
decrease an intensity, a repetition rate, and/or a pulse width of
the light emitted by the light source 18 if it is determined that
the sensor is not on the patient's skin or has become separated
from the patient's skin. In some embodiments, the system 8 may
provide an indication that the sensor 10 is not on the patient's
skin (e.g., an audible or visual alarm, a message on the display
14) and/or may provide instructions to the user to correct the
issue, such as instructions to adjust the sensor, to firmly press
the sensor to the patient's skin, to replace a disposable portion
of the sensor 10, to replace all of the sensor 10, or to clean the
patient contact surfaces, for example.
[0052] In certain embodiments, if it is determined that the sensor
10 is on the patient's skin, then the system 8 may turn on the
light source 18 as shown in step 186. In some embodiments, the
light source 18 may initially run at low power or at a low
repetition rate (e.g., about 50 Hz, 100 Hz, etc.) when the sensor
10 is turned on and/or prepared for placement on the patient, and
the system 8 may increase the intensity, the repetition rate,
and/or the pulse width of the light emitted by the light source 18
when it is determined that the sensor 10 is applied to the
patient's skin. In some embodiments, the system 8 may provide an
indication or confirmation that the sensor 10 is applied to the
patient's skin (e.g., an audible or visual alarm, a message on the
display 14).
[0053] FIG. 8 is a flow diagram of another technique for operating
the sensor 10 and/or for controlling the light source 18 in
accordance with the present disclosure. In certain embodiments, the
appropriate response may depend on a type of PA spectroscopy system
8 and/or a current mode of operation of the system 8. Accordingly,
in some embodiments, the system 8 may additionally determine the
type of system 8 (e.g., a system configured to monitor oxygen
saturation, a system configured to monitor indicator dilution,
etc.), and/or the current mode of operation of the system 8 (e.g.,
the current mode of operation of a system having multiple modes,
such as a default mode, an indicator dilution mode, etc.). Such a
determination may be made by accessing information (e.g., operating
parameters or capabilities, sensor identification data, such as
sensor model number, etc.) stored in the sensor 10, such as in the
encoder 28. In some embodiments, the current monde of operation may
be input by a user. As shown in FIG. 8, at step 190, a signal
indicative of whether the sensor 10 is positioned on the patient's
tissue is received (e.g., at the microprocessor 32). The signal may
be generated by the sensing elements 26, by the optical detector
22, or by the acoustic detector 20, for example. At step 192, the
system 8 processes the signal to determine whether the sensor 10 is
positioned on the patient's skin. As shown in step 194, if the
sensor 10 is not applied to the patient's skin, the system 8 may
turn the light source 18 off or may adjust the laser power, pulse
rate, and/or pulse width so that the light is emitted at an
eye-safe level. Such adjustments may be carried out by controlling
the drive current of the laser or by controlling the AOM 25, for
example. The system 8 may additionally or alternatively take any
another suitable action, such as providing an indication that the
sensor 10 is not on the patient's skin. As discussed above, the
system 8 may also provide instructions, such as instructions to
adjust the sensor, to firmly press the sensor to the patient's
skin, to replace a disposable portion of the sensor 10, to replace
the whole sensor 10, or to clean the patient contact surfaces, for
example.
[0054] If the signal indicates that the sensor 10 is applied to the
patient's tissue, the system may proceed to step 196. At step 196,
the system 8 may determine a current mode of operation of the
sensor 10. For example, the system 8 may determine whether the
system 8 is operating in a default PA mode, an indicator dilution
mode, or any other suitable mode. If the sensor 10 is determined to
be on the patient's skin and the system 8 is operating in an
indicator dilution mode as shown in step 198, the system 8 may
increase the repetition rate of the light emitted by the light
source 18 as shown in step 200. For example, the sensor 10 may
initially run at a very low repetition rate in preparation for
patient monitoring, and the system 8 may increase the repetition
rate (e.g., from about 50 Hz-100 Hz to about 1 kHz) if it is
determined that the sensor 10 is on the patient's skin. However, if
the sensor 10 is determined to be on the patient's skin and the
system 8 is operating in a default PA mode as shown in step 202,
the system 8 may increase the power of the light emitted by the
light source 18 as shown in step 204. For example, the drive
current may be increased to increase the power of the light.
[0055] It should be noted any of the methods provided herein, may
be performed as an automated procedure by a system, such as system
8. Although the flow charts illustrate the steps in a certain
sequence, it should be understood that the steps may be performed
in any suitable order and certain steps may be carried out
simultaneously, where appropriate. For example, with reference to
FIG. 8, the system 8 may first determine a current mode of
operation of the sensor 10, and then proceed to determine whether
the signal indicates that the sensor is applied to the patient's
tissue. Further, certain steps or portions of the methods may be
performed by separate devices. In addition, insofar as steps of the
methods disclosed herein are applied to the received signals, it
should be understood that the received signals may be raw signals
or processed signals. That is, the methods may be applied to an
output of the received signals.
[0056] Certain described embodiments relate generally to features
that generate signals indicative of whether the sensor 10 is
applied to the patient's skin. It should be understood that in
other embodiments, the sensor 10 may additionally or alternatively
include other features, such as a switch. The switch may be a
mechanical switch and may be positioned at any suitable location on
the sensor 10. For example, the switch may be positioned on a
bottom surface of a spacer, such as the bottom surface 128 of the
spacer 126 of FIG. 3. The switch may prevent the light source 18
from emitting light unless the sensor 10 is applied to the
patient's skin. More particularly, the switch may be closed when
the sensor 10 is applied to the patient's skin and open when the
sensor 10 is not applied to the patient's skin, or vice versa.
Thus, for example, the switch may mechanically disable the light
source 21 when the sensor 10 is not applied to the patient's skin,
and thus may be a light-controlling feature.
[0057] The disclosed embodiments are provided in the context of a
photoacoustic spectroscopy system. However, it should be understood
that the features described herein may be incorporated into any
sensor assembly having a high-intensity light source. Furthermore,
the various features and techniques described herein may be
combined or utilized together in any suitable manner to determine
whether a medical sensor, such as a photoacoustic sensor, is
applied to the patient's skin and to appropriately control the
light source. While the disclosure may be susceptible to various
modifications and alternative forms, specific embodiments have been
shown by way of example in the drawings and have been described in
detail herein. However, it should be understood that the
embodiments provided herein are not intended to be limited to the
particular forms disclosed. Rather, the various embodiments may
cover all modifications, equivalents, and alternatives falling
within the spirit and scope of the disclosure as defined by the
following appended claims.
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